1. Field of the Invention
The present invention relates in general to photolithography, and particularly to focus method during exposure.
2. Description of the Related Art
Photolithography, comprising priming, photoresist coating, baking, exposure, post bake, and development, is used to transfer specific patterns on semiconductor devices or integrated circuits during the fabrication process. A masking step transfers the pattern of a photomask on a photoresist layer on the device surface by exposing the photoresist through the mask. Selected areas of the photoresist, based on the pattern of the mask, are then etched so that subsequent process steps, such as impurity introduction, oxidation, and metallization, can be performed. A semiconductor device with the desired electrical properties is then obtained after several of the photolithography and etching steps. The critical factor of the yield of the semiconductor devices depends on the photolithography.
Because of further increases in density of circuit pattern of LSI and further reduction in line width of patterns to be transferred, into a submicron order, in exposure apparatuses to be used in a lithographic process for LSI manufacture it is very important to accurately set the best focus position, namely, best exposure conditions, to keep the resolving power of a projection lens constant.
Conventional focus method is performed using a test mask having a rhomboid transparent area and an exposure apparatus to determine the best focus position by the user until the exposed image is almost rhomboid.
However, it is not accurate for users to determine the best focus position. Additionally, it is time consuming to change from the test mask to the process mask after focusing.